1. Field of the Invention
The present invention relates to an orbit antenna for a nuclear magnetic resonance (NMR) imaging apparatus. Although a key application of the antenna is in the medical field for examination of patients' eyes, the invention is also useful for viewing different body locations when the part or organ to be examined is of small size.
2. Description of the Prior Art
In an NMR imaging device, a body to be examined is subjected to a constant high-strength magnetic field B.sub.O. A radiofrequency excitation is additionally applied to the patient's body when it is under the influence of this field, thereby inducing resonance of oscillation of magnetic moments of particles within the body. After excitation, a measurement is performed on the resonance signal emitted as feedback by the particles when their magnetic moments tend to re-align themselves with the field B.sub.O. The measured signal is processed in order to extract therefrom cross-sectional images of tissues or organs under examination.
The antenna in accordance with the invention has the essential function of collecting or detecting the resonance signal to be measured and is of the so-called surface type. Surface antennas are receiving antennas which are placed on the bodies to be examined. They are different from the transmitting antennas which are usually attached to the imaging apparatus itself. Since they are located in closer proximity to the body to be examined than antennas which are attached to the apparatus, surface antennas receive a signal which achieves a higher quality standard or signal-to-noise ratio. One of the main problems set by the construction of antennas lies in their symmetrization. This means that all parts of the antenna have to make an equal contribution to formation of the detected signal. It is a known fact that, if the antenna is unsymmetrical, it receives objectionable noise signals which are external to the body to be examined.
The known solutions of symmetrization of antennas involve the use of magnetic coupling devices which are known in other technical fields and serve to balance the contributions of the different portions of the antenna to formation of the detected signal. Unfortunately, the presence of ferromagnetic materials is prohibited in an NMR device. In practice, uniform distribution of magnetic fields in a device of this type is a strict requirement which governs the quality and fidelity of images. Moreover, the magnetic couplings produced by the devices mentioned above have another function, namely that of matching the impedance of the antenna with the impedance of a high-frequency line which serves to carry the detected signal. The problem to be solved is therefore that of replacing these couplings by other means.